The invention pertains to the field of electronic control of fluid flow, and, more particularly, to the field of integrated, microminiature electric to fluidic valves where the flow of a gas or fluid may be controlled by an electronic signal from some control logic.
Many industrial machines and industrial or manufacturing facilities are pneumatically powered. Pneumatic power provides very efficient actuation of machines, and is frequently used in robot machines for assembly line work. These types of machines are frequently controlled by computers or other logic circuitry. The logic circuitry decides the sequence of events that needs to occur, and generates electrical signals to cause same to occur as planned. When the sequence of events involves physical movement of portions of the machines which are driven pneumatically, there arises a need for a valve or conversion device which can convert the electrical control signals from the control logic into pneumatic control signals to drive the machine parts.
Since such machines often use many moving parts which are controlled by numerous individual pneumatic lines, it is frequently found that many such electric to fluidic valves are necessary. In such environments, the electric to fluidic valves need to be cheap, reliable, power efficient, small, and compatible with electronic interface circuitry between the valve and the computer or control logic.
In very precise robotic movement applications or other applications where very precise movement control is necessary, it is necessary to have precise control of the shape of the pneumatic control drive pulses. In other applications, such as gas chromatography, the shape of the fluid pulses entering the column must be precisely controlled to get precision assay data from the column. In either of these types of applications, the valves used to control the fluid flow must be precision valves which have little or no dead volume. Dead volume is the unknown volume which is trapped in a valve when it makes a transition from open to closed. This trapped fluid may escape into the stream thereby causing the shape of the fluid pulse to be altered from the desired shape. For example, in typical gas chromatograph systems, if a valve is used which has dead volume the edges of the output fluid pulse entering the separation column (in terms of the the volume of gas flowing at any particular instant in time) may not be vertical or sharply defined. Likewise, for precise robotic movement, it is desirable to have very sharp cut-offs for the pneumatic pulses used to drive robot fingers and arms to get precise positional control for the movement.
One known way of controlling the flow of a fluid using an electrical pulse is the electric to fluidic valve developed by Steve Terry of Stanford University. This valve uses a substrate such as silicon which has a thin membrane machined therein. This cavity is formed by the etching a hole almost completely through the substrate. This leaves a thin bottom wall for the cavity which is used as a flexible membrane. Attached to the side of the first substrate in which the membrane is formed is a second substrate which has a manifold type cavity etched therein with a passageway or nozzle formed in a wall of the manifold cavity for entering or escaping gas. The manifold cavity also has other ports formed therein to complete a fluid path into the manifold and out the nozzle or vice versa. The manifold cavity in the second substrate is positioned over the membrane of the first substrate such that when the manifold of the first substrate is flexed, it contacts a sealing ring formed around the nozzle of the manifold cavity thereby closing off the fluid flow path between the nozzle and the other ports into the manifold cavity. With the membrane of the first substrate in an unflexed position the nozzle in the manifold cavity would not be pinched off, and fluid would be free to flow through the input port and the manifold cavity and out through the nozzle or vice versa. The membrane of the first substrate is forced to flex by mechanical forces exerted thereon by a piston. This piston is driven by a solenoid or other type of electromagnetic device.
One disadvantage of the above described valve configuration is that the solenoid requires a high power source, and is a large power consumer. Further, the solenoid or other electromagnetic device is large and heavy. The cavities in the first and second substrates could be formed with much smaller dimensions if it were not for the fact that the solenoid is large. Because the first and second substrates are silicon wafers which are etched using conventional planar photolithography techniques, it would be possible to make the electric to fluidic valve much smaller in dimension were it not for the solenoid. Such a prior art electric to fluidic valve construction is inefficient in its use of space. Because the solenoid is mechanically attached to the first substrate such that the piston of the solenoid pushes against the membrane in the first substrate and because the solenoid is large enough to consume much of the wafer space, generally only three such valve structures can be formed on a single silicon wafer. Such a structure is relatively expensive to build, and the bond between the solenoid and the glass is difficult to make. Generally, the solenoid is attached to a thick pyrex wafer by nuts and bolts. This form of attachment is both expensive to fabricate and a major source of failures. Further, such a structure has a moving part which can be another source of failure. The principal defect of such a structure, however, is the fact that the entire structure cannot easily be mass produced with planar lithography techniques. This is because the solenoid can not be manufactured by such techniques.
Another system which has been used in the past in the field of ink jet printing uses a principle used in the invention involving the tendency of fluids and gases to expand and to create higher pressures in a cavity when heated. The particular system which embodies this principle is a Hewlett packard ink jet printer. This printer structure uses a print head which has a small cavity formed in or over a substrate. The substrate has formed thereon a resistive element, and the cavity is located over the resistive element. The cavity has a small ink jet nozzle therein through which ink may escape in small droplets when the pressure of ink in the cavity rises above the atmospheric pressure. In operation, such a structure will shoot out an ink drop each time a heating pulse is applied to the resistive element. The heat from the resistive element raises the temperature of the ink in the cavity thereby causing its vapor pressure to increase according to the laws of thermodynamics. When the pressure of the ink inside the cavity rises, one or more ink droplets are forced out of the cavity through the ink jet port in the cavity wall. Such a structure is an example of an unrelated application of a principle of thermodynamics which is used in the invention. As far as the applicant knows, no such application of the principle of expansion of a fluid in a confined cavity with increasing temperature has ever been used to control a fluid valve.
Thus a need has arisen for an electric to fluidic valve which may be mass produced cheaply using conventional planar lithography techniques, which does not use large amounts of energy, which is small and efficiently uses wafer space, which has no moving parts which slide across each other, which has sharp cutoff characteristics with little or no dead volume and which is compatible with the formation of interface or driver circuitry on the same silicon wafer in which the electric to fluidic valve is formed.